(1) Field of the Invention
The present invention relates to a solid-state imaging device.
(2) Description of the Related Art
Demand for video cameras and digital still cameras tends to increase year by year. With increase in demand, clientele is broadened. There is, a demand for a wide dynamic range of images not only from a conventional clientele but also from a new clientele. Thus, development that meets customers' needs is required for a solid-state imaging device, particularly, a charge coupled device (CCD) solid-state imaging device, to be used in a camera.
A typical CCD image sensor will be described. FIG. 1 shows a configuration of a typical CCD image sensor. The CCD image sensor includes: plural photodiodes 901 arranged in a matrix; vertical CCDs 903, each of which is a charge transfer path in a vertical direction arranged on a left side of each row of the photodiodes 901; a horizontal CCD 907 which is a charge transfer path in a horizontal direction arranged at lower ends of the rows of the vertical CCDs 903; and an output unit 908 that outputs a voltage value corresponding to charges to outside of the CCD image sensor. A photodiode 901 converts incident light to a charge and the corresponding transfer gate 902 transfers the photoelectrically converted charge to the corresponding vertical CCD 903. The vertical CCD 903 transfers the transferred charge to the horizontal CCD 907. The horizontal CCD 907 transfers the charge transferred from the vertical CCD 903 to the output unit 908. The output unit 908 converts the charge to a voltage and outputs the obtained voltage value to outside of the CCD image sensor.
Now, conventional methods for improving a dynamic range will be described. There are three conventional methods for improving a dynamic range and thus will be described in turn.
First, a first conventional method for improving a dynamic range (see, for example, Japanese Laid-Open Patent Application No. 11-234575) will be described using FIG. 2. FIG. 2 is a diagram for describing the first method. A difference between a CCD image sensor of FIG. 2 and the CCD image sensor of FIG. 1 is as follows. In the CCD image sensor of FIG. 1, one pixel includes one photodiode 901; on the other hand, in the CCD image sensor of FIG. 2, a pixel 909 includes two photodiodes 909a and 909b located above and below. The sensitivity of a pixel A corresponding to the photodiode 909a is different from that of a pixel B corresponding to the photodiode 909b. The sensitivity of the pixel A is lower than that of the pixel B. In the CCD image sensor of FIG. 2, the pixel 909 includes a pixel A and a pixel B located above and below. A signal from the pixel A is stored in a memory 911. A signal from the pixel B is directly outputted to an adding circuit 913 and added, by the adding circuit 913, to the signal from the pixel A previously stored in the memory 911, and then the added signal is outputted.
FIG. 3 shows characteristic curves of a signal charge relative to incident light in the first method. A signal charge obtained as a whole is a charge to be obtained by adding together a signal charge within a luminance range of a pixel A and a signal charge within a luminance range of a pixel B. As shown in FIG. 3, when one pixel includes a pixel A and a pixel B, the dynamic range is improved as compared with the case where one pixel includes only a pixel B.
Next, a second conventional method for improving a dynamic range (see, for example, Japanese Laid-Open Patent Application No. 2000-92395) will be described using FIG. 4. FIG. 4 is a diagram showing pulse waveforms of a VSUB voltage and a read voltage for improving the dynamic range. A period between charge accumulation times t1 and t2 in a field period of a first field is shorter than a period between charge accumulation times t3 and t4 in a field period of a second field, and the sensitivity of one same pixel is lower in the first field than in the second field.
FIG. 5 shows characteristic curves of a signal charge relative to incident light in the second method. A signal charge obtained as a whole is a charge to be obtained by adding together a signal charge within a luminance range of the first field and a signal charge within a luminance range of the second field. As shown in FIG. 5, when one frame includes a first field and a second field, the dynamic range is improved as compared with the case where one frame includes only a second field.
Finally, a third conventional method for improving a dynamic range (see, for example, Japanese Laid-Open Patent Application No. 2001-86402) will be described using FIG. 6. FIG. 6 is a diagram showing a characteristic curve of a signal charge relative to incident light in the third method. The signal charge linearly increases relative to the incident light up until the point just before a knee point. When an amount of the incident light exceeds the knee point, the signal charge logarithmic-functionally increases. Hence, in the third method, by converting the signal charge that logarithmic-functionally increases into a signal charge for the case where the signal charge is assumed to linearly increase, the dynamic range is improved. FIG. 6 shows a state in which an actual signal charge c is converted to a signal charge c′.
In the first method, however, since one pixel includes two pixels, the number of effective pixels is reduced, degrading resolution. Normally, cells of an imaging element are formed in a square or an approximately square shape such that the number of the cells is equal to the number of divisions in a vertical direction and a horizontal direction of an image to be reproduced later (and that resolutions in the vertical direction and the horizontal direction are equal). However, when the first method is used, since one pixel is formed by two pixels adjacent to each other in the vertical direction or the horizontal direction, the number of effective pixels is not equal to the number of divisions in the vertical direction or the horizontal direction (a resolution in the vertical direction or the horizontal direction) which is obtained upon image reproduction. Generally, the resolution of the human eyes strongly depends on a lower resolution, and thus, humans perceive resolution degradation. In order to obtain equal resolutions in the vertical direction and the horizontal directions, a method may be considered in which pitches in the vertical direction and the horizontal direction are made equal with two adjacent pixels as one pixel; however, this method causes a problem that the amount of saturation charges in one pixel is extremely small.
Next, in the second method, a reduction in the number of effective pixels that occurs in the first method does not occur. However, since a high-sensitivity state and a low-sensitivity state are created in a time series, upon imaging a moving object or the like, the difference in time upon obtaining information in a high luminance range and information in a low luminance range in one same pixel becomes a problem. That is, in the second method, humans see an image as if the image were blurred. When this moving image is viewed as a still image, a state in which the image is blurred becomes more apparent.
Finally, in the third method, the problem of a reduction in the number of effective pixels that occurs in the first method or the problem of blurring of an image that occurs in the second method does not occur. However, when, for example, resist shapes upon implanting impurities for forming photodiodes or the like are slightly different between pixels at the center and pixels at the periphery, variations occur in implantation density, and as a result, the amount of saturation charges in a photodiode varies from pixel to pixel. This causes a phenomenon that in a knee range the amount of accumulated charges varies from pixel to pixel. In the knee range, as described above, the signal charge logarithmic-functionally increases as the amount of incident light increases. Therefore, conversion efficiency for converting from the amount of accumulated charges to a signal voltage needs to be enhanced in a logarithmic range rather than in a linear range, and consequently, the difference in the amount of accumulated charges appears as image inconsistency at high luminance.